Internal combustion engines with inlet valves, the valve lift of which can be adjusted continuously or discretely by means of an adjustment mechanism, are used with increasing frequency in vehicles, see for example DE 195 20 117 and MTZ Motortechnische Zeitschrift (Automotive Engineering Magazine) 61 (2000) 11, pages 730-743. In this case, the adjustment mechanism can become defective resulting in the valve lift obtained not as expected. This error results in an increase in pollutant emissions and a corresponding discrepancy between the required torque and the torque actually delivered. This situation can be identified using a valve lift sensor. However an additional sensor of this kind requires a corresponding outlay.